Re: [PATCH 2/9] KVM: x86: simplify read_emulated

Marcelo Tosatti
2012-07-20T19:52:12Z

On Fri, Jul 20, 2012 at 09:15:44PM +0800, Xiao Guangrong wrote:
> On 07/20/2012 06:58 PM, Marcelo Tosatti wrote:
> > On Fri, Jul 20, 2012 at 10:17:36AM +0800, Xiao Guangrong wrote:
> >> On 07/20/2012 07:58 AM, Marcelo Tosatti wrote:
> >>
> >>>> - }
> >>>> + rc = ctxt->ops->read_emulated(ctxt, addr, mc->data + mc->end, size,
> >>>> + &ctxt->exception);
> >>>> + if (rc != X86EMUL_CONTINUE)
> >>>> + return rc;
> >>>> +
> >>>> + mc->end += size;
> >>>> +
> >>>> +read_cached:
> >>>> + memcpy(dest, mc->data + mc->pos, size);
> >>>
> >>> What prevents read_emulated(size > 8) call, with
> >>> mc->pos == (mc->end - 8) now?
> >>
> >> Marcelo,
> >>
> >> The splitting has been done in emulator_read_write_onepage:
> >>
> >> while (bytes) {
> >> unsigned now = min(bytes, 8U);
> >>
> >> frag = &vcpu->mmio_fragments[vcpu->mmio_nr_fragments++];
> >> frag->gpa = gpa;
> >> frag->data = val;
> >> frag->len = now;
> >> frag->write_readonly_mem = (ret == -EPERM);
> >>
> >> gpa += now;
> >> val += now;
> >> bytes -= now;
> >> }
> >>
> >> So i think it is safe to remove the splitting in read_emulated.
> >
> > Yes, it is fine to remove it.
> >
> > But splitting in emulate.c prevented the case of _cache read_ with size
> >> 8 beyond end of mc->data. Must handle that case in read_emulated.
> >
> > "What prevents read_emulated(size > 8) call, with mc->pos == (mc->end - 8) now?"
>
> You mean the mmio region is partly cached?
>
> I think it can not happen. Now, we pass the whole size to emulator_read_write_onepage(),
> after it is finished, it saves the whole data into mc->data[], so, the cache-read
> can always get the whole data from mc->data[].

I mean that nothing prevents a caller from reading beyond the end of
mc->data array (but then again this was the previous behavior).

ACK